A subscript operator must be defined as a class member function.
To appear on the left-hand side, its return value must be an lvalue. This is achieved by specifying the return type as a reference:
```
inline char&
String::operator[]( int elem ) const
{
   return _string[ elem ];
}
```
If the return type of the operator= is a reference, an object defined by T obj = arry[3] can change the value of arry[3], ended up causing unexpected behavior. True? False. T obj = arry[3] calls copy ctor, T obj; obj = arry[3] calls copy assignment.